The exhaust gas emitted from an internal combustion engine, particularly a diesel engine, is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that constitute particulate matter (“PM”). Catalyst compositions, typically disposed on catalyst supports or substrates, are provided in an engine exhaust system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.
One type of exhaust treatment technology for reducing CO and HC emissions is an oxidation catalyst device (“OC”). The OC device includes a flow-through substrate and a catalyst compound applied to the substrate. The catalyst compound of the OC induces an oxidation reaction of the exhaust gases once the OC device has attained a threshold or light-off temperature. One type of exhaust treatment technology for reducing NOx emissions is a selective catalytic reduction (“SCR”) device that may be positioned downstream of the OC device. The SCR device includes a substrate, having a SCR catalyst compound applied to the substrate. A reductant is typically sprayed into hot exhaust gases upstream of the SCR device. The reductant may be a urea solution that decomposes to ammonia (“NH3”) in the hot exhaust gases and is absorbed by the SCR device.
The ammonia and the NOx in the exhaust gas are converted into an intermediate such as, for example, ammonium nitrate (“NH4NO3”) that is stored on the SCR device until the SCR device attains a threshold or light-off temperature. After light off, the ammonia reduces the NOx to either nitrogen or nitrous oxide (“N2O”) in the presence of the SCR catalyst. When the intermediate decomposes, if more nitrogen oxide (“NO”) is present in the exhaust gas than nitrogen dioxide (“NO2”), then the intermediate decomposes into nitrogen. If more nitrogen dioxide is present in the exhaust gas than nitrogen oxide, then the intermediate decomposes into nitrous oxide. However, nitrous oxide is a greenhouse gas. Accordingly, it is desirable to provide an efficient approach for limiting the amount of NOx that is reduced into nitrous oxide by the SCR catalyst.